In recent years, USB (Universal Serial Bus) interfaces are becoming established as interfaces for PC peripheral equipment due to their good connectivity such as hot plugging capability that allows an external drive to be connected to or disconnected from a personal computer (PC) with power being turned on. USB-compliant peripheral equipment has extended from low and medium speed transmission functions such as mouses, keyboards, printers, scanners, etc., of early days up to middle and high speed transmission functions such as FDD (Floppy Disk Drive), HDD (Hard Disk Drive), MO (Magneto Optical disk), CD-ROM (Compact Disk ROM), AV equipment, etc.
One of the functions of USB or IEEE 1394 is to supply electric power from a PC main body to its peripheral equipment. The peripheral equipment, being compatible with these connection standards or specifications and being bus-powered as well can be supplied with electricity from the PC main body. In other words, a special power supply need not be separately connected to the peripheral equipment itself. A USB connector can be four pins, of which two are used for signal lines for data transmission and the remaining two are used for supplying electric power to the peripheral equipment or USB device.
As the number of pieces of peripheral equipment connected to a PC, there arise various problems such as a shortage of outlets, the inability of use caused by forgetting to turn on power, and so on. In addition, it might happen that a notebook PC, if used outdoors, can itself be powered by a battery but can not actually be used because of non-availability of power to its peripheral equipment. In such cases, however, these problems can be solved if the peripheral equipment is bus-powered. In recent years, bus-powered products are coming to appear even in the fields of information storage devices or information recording and reproducing devices such as FDDs, Super Disk drives, ZIP drives, etc.
According to the USB standard, it is specified that the power supply voltage output from a port of the PC main body should have a voltage of 5 V±5% and a minimum current supply capacity of 500 mA. Accordingly, the specification of peripheral equipment should be in these ranges, and the consumption current of a high-power bus-powered device is specified as a maximum of 500 mA.
However, when an information storage device draws in the current supplied, the power supply voltage supplied to the information storage device drops considerably due to the direct current resistance of its cable and the contact resistance of its connector as compared with the power supply voltage of the USB in the PC main body. Therefore, in the worst case, there arises a problem that a reset circuit in the information storage device might be operated, thus making its recording and/or reproducing operation unstable or incomplete. Also, there might occur another problem that the information storage device is separated or cut off from the PC host at a time. Moreover, there might arise a further problem that the power supply for the information storage device is turned off depending upon the threshold of a regulator IC in the PC main body with an overcurrent protective function.
The present invention is intended to obviate the problems as referred to above, and has for its object to provide an information storage device which can be used in a stable manner in various connection environments, and which can eliminate or avoid resetting during operation.